Conceptual
Approach to Light Limitation
It
has been shown that the light environment during the
growing season is the most important period determining
survival of seagrasses (Moore et al. 1997; Dixon 2000,
Batiuk et al. 2000). The amount of light required
for growth and reproduction (flowering) is the cumulative
light received during the growing period of the plant's
life history, typically spring and summer months for
temperate seagrasses. Light attenuation by the water
column is the major variable related to seagrass decline.
Low light levels, below some minimum physiological
requirement (typically 15-25% of incident surface
light = Io), usually results in a loss of seagrasses.
Light is attenuated down the water column resulting
in less light available at the bottom (Iz) than at
the surface (Io). Factors that contribute to light
attenuation can include (Fig. 1):
1. turbidity, expressed as total suspended particulate
matter (SPM),
2.
phytoplankton, which both absorb and scatter light,
expressed in chlorophyll concentration (Chl a),
3.
colored dissolved organic matter (CDOM) leaching from
decaying vegetation and peat deposits,
4.
macroalgae and epiphytic microalgae that grow on the
seagrass. These are usually most problematic when
eutrophication is taking place.
One
of the goals of our preliminary research has been
to determine the importance of each of these factors
on light attenuation to seagrasses in North Carolina
during different seasons. We have established a collaborative
effort with Charles Gallegos (Smithsonian Environ.
Research Ctr) to refine a bio-optical water quality
model he has developed for North Carolina conditions.
The first objective in bio-optical modeling is to
determine the contribution of different substances
in the water to the spectral absorption and scattering
coefficients. Light absorption in water is the sum
of contributions due to water itself, colored dissolved
organic matter (CDOM), phytoplankton pigments, and
non-algal particulate matter which consists of mineral
and detrital particles, heterotrophic plankton, and
the non-pigmented portion of algalcells.
Absorption spectra by different components exhibit
characteristic shapes, which are determined by measuring
the absorption by different components in isolation.
The absorption by the different components
is normalized by its relevant water quality measure,
to determine the specific-absorption spectrum of
each component.
Specific-absorption spectra are a measure
of the incremental effect of a unit change in concentration
of a parameter on the total absorption spectrum.
Absorption by phytoplankton is normalized
to Chl a, absorption by non-algal particulates
is normalized to the concentration of total suspended
solids (TSS), and absorption by CDOM is normalized
to its value at 440 nm.
This decomposition allows us to express the
total absorption spectrum, at(l), as a sum of the 4 components,
(1) where l=wavelength, aw(l) is the absorption by pure
water, aX*(l) are the specific-absorption spectra of CDOM
(X=CDOM), phytoplankton (X=f), and non-algal particulates (X=p-f); the scale factors for the
components are the absorption by CDOM at 440 nm, (aCDOM(440),
the concentration of chlorophyll, [CHL], and the concentration
of suspended particulates, [TSS].
A similar procedure is used to model scattering
coefficient as a function of TSS. The total absorption and scattering spectra
may then be used with any of a number of radiative
transfer programs available to predict the penetration
of light underwater.
The bio-optical model is useful because it permits
us to determine the relative contributions of the
different water quality parameters to light attenuation
at different positions in the estuary. Comparing light attenuation calculated at the
deep survival limit of seagrasses with water quality
concentrations measured there allows the determination
of ranges of water clarity that permit expansion or
cause contraction of the seagrass bed. Basing the model on inherent optical properties has the advantage
that extrapolation beyond the range of water quality
concentrations encountered during model development
is possible, because the absorption and scattering
coefficients are linearly related to the relevant
water quality concentrations (Fig. 1).
Such an exercise can be used to determine the
availability of light at the edge of the seagrass
bed in response to hypothetical scenarios, such as
accelerated eutrophication resulting from increased
nutrient loading in the watershed. This bio-optical
model has already been calibrated to conditions typical
for Chesapeake Bay, MD (Gallegos 2001) and Indian
River Lagoon, Fla (Gallegos and Kenworthy 1996), estuaries
with significant seagrass habitats.
Figure
1: A (left) Conceptual diagram of light attenuation
down the water column (PLW), and the relative contributions
of turbidity, chlorophyll, and color to attenuation,
resulting in significantly reduced light at depth. Additional
attenuation can occur at the leaf surface due to epiphyte
fouling (PLL), which occurs primarily under heavily
eutrophic conditions. 